Hubble's Theories

The Island Universe Theory was developed by Hubble in the pursuit of understanding nebulas: clouds of dust and gas within galaxies. Before Hubble’s time, Scientists were unable to tell the difference between the distances between nebulas, and other galaxies. To try to determine the distance from earth, Cepheid Stars: yellow stars which go through brightness cycles, are used by scientists to determine an approximate distance from the earth to the star.

Using the Cepheid Stars within Nebulas, Hubble discovered the Andromeda Nebula in 1923. After many calculations it was determined that Andromeda (now known as the Great Andromeda Spiral Galaxy) was in fact over 800 000 light years away from the boundaries of the Milky Way Galaxy. This was the basis of which Hubble proved the Existence of other galaxies in the universe. Further research found more Nebulas and many more galaxies, thereby proved Hubble’s Island Universe Theory.

The Universal Expansion Theory is based on the fact the universe is still expanding. In his previous experiments, Hubble realized that the galaxies around the earth were moving away from it. While observing this movement Hubble measured the wavelengths of light from the galaxies. The visual light spectrum shows us that longer wavelengths have a red colour, while shorter wavelengths have a blue colour. With this in mind, if the wavelengths were red, than the galaxy observed is traveling away from the earth, while a bluish colour indicates the galaxy moving towards the earth. The reason for this can be explained by using the Doppler Shift, which involves the pitch of sound made by an object (frequency), as it moves further away or closer to an observer. As the source of the sound moves closer to the observer, a lower pitched sound is heard. This applies to light as well, if the astronomical object is moving away from the earth, than a reddish colour is observed, as the wave length is longer. Speed is also a component of the Doppler Shift, when analyzing sound. Because the wave length is longer (closer to the red end of the light spectrum), the galaxy is moving away from the Milky Way. The Doppler shift, and therefore the speed of the galaxy is greater for more distant galaxies. The constant relationship between the speed and distance is evidence for the expansion of the universe. The relationship between the speed of expansion and the distance is Hubble’s Law.

Hubble’s Law incorporates the density, expansion rate and age of the universe. The density establishes how much the force of gravity will slow the rate of expansion. This expansion rate is dependant not only on the density, but also the age of the universe. The expansion rate can be measured by estimating the density of the universe, and measuring the red shifts to find expansion rate, then the age of the universe can be determined as an estimate. The expansion rate of the universe can be calculated by determining the relationship between the distance of the object from earth, and the speed at which it is moving; this is known as Hubble’s constant. A formula is developed where the velocity of the object can be measured by multiplying the distance with Hubble’s constant. If the constant is relatively large, than the universe is expanding relatively quickly.